I've collected a bit of MPG information of using a teardrop that might be interesting for folks considering one.
My 2016 4Runner gets ~17 MPG for all-around use. I'm retired and don't commute. So some of this is highway, some around town, and some rural 2-lane.
Last September, I drove to Indianapolis to pick up my Hiker Trailer 5X8 Deluxe (the non-off road version.) It happened to be a weekend with relatively no wind. I filled up at home, drove ~350 miles to Indianapolis, and filled up again. I got 17.6 MPG (actual calculation, not the truck's computer.) I then picked up the trailer, drove home, and filled up again. This trip I got 16.7 MPG, or a 0.9 MPG hit for the empty trailer. Being lower and narrower than the 4Runner, it didn't need to push through the wind very much. This was all Interstate at about 65 MPH on flat ground.
A month later, I mounted my RTT on top of the trailer, and pulled it to my brother's barn, about 150 miles away. Again, a relatively wind free day, and a full tank. I got 13.4 MPG on this trip! There was maybe 200# of stuff in the trailer, but the biggest hit was the big brick on top of the trailer catching the wind. I'd have thought that far back, it would be in the truck's wind shadow, but apparently not. The return trip home yielded ~17 MPG (I didn't fill up for a couple of days, so don't have the exact numbers.) This was on rural 2-lane roads at about 50-55 MPH on flat ground.
Now, I realize that for off road use, MPG difference would likely be insignificant. But for me, being 300-2000 miles from any place interesting to explore, 3 MPG makes a big difference. Last year's 7000 mile road trip would have made about $200 difference in fuel cost.
As always, YMMV